Polyposis coli provides an excellent model for the study of both adenomas and cancers of the large bowel. Antioxidants retard experimental carcinogenis and the administration of an antioxidant, ascorbic acid, to a few patients with polyposis coli was associated with a reduction in the number and size of adenomas in some cases. In the proposed investigation, a randomized double blind study of ascorbic acid or placebo will be performed in approximately 40 polyposis patients at St. Mark's Hospital, London. U. K. St. Mark's Hospital is the only hospital in the world with access to a sufficient number of patients for a definite phase III study, the long term objective of which is the chemoprevention of large bowel cancer.